(a) Field of the Invention
The present invention relates to a diesel engine. More particularly, the present invention relates to a method for controlling exhaust gas recirculation (EGR) of a diesel engine.
(b) Description of the Related Art
Typically a diesel engine is provided with an exhaust gas recirculation (EGR) system that recirculates a part of an exhaust gas into an intake system such that the temperature of burning gas in the combustion chamber can be lowered so as to reduce the generation of NOx.
While an engine is driven, a target air amount is set depending on driving states, and an optimal mixture of the recirculated exhaust gas and the fresh air is obtained by controlling an EGR duty ratio for controlling an EGR valve in the EGR system. Typically, the optimal values for controlling the EGR valve depending on driving states are preset as a map table, and then the EGR valve is controlled according to the map table considering a current engine state of the engine.
A diesel engine may be provided with a turbocharger in order to increase an output power of the engine, to reduce the emission, or to enhance fuel consumption. In a typical diesel engine with a turbocharger, a high-temperature and high pressure exhaust gas before the turbocharger is partially taken to be recirculated.
According to such a method, an EGR gas is of a very high temperature even if it may be passed through an EGR intercooler, and a raw gas exhausted from the engine is forwarded to the engine. Therefore, only limited effect of reducing emission may be obtained. For example, such a method may be used to satisfy a Euro 4 regulation, however, it will be very difficult to satisfy stricter emission control regulations such as Euro 5, Euro 6, and US Tier II Bin 5 with such scheme.
Recently, a low pressure EGR system is under the investigation in order to satisfactorily reduce the emission of the diesel engine. According to the low pressure EGR system, the exhaust gas is typically taken after a diesel particulate filter (DPF) to be recirculated into the engine.
In such a scheme, the length of the path of the EGR gas becomes longer in comparison with a typical high pressure EGR system, and thus the pressure of the EGR gas that is returned to the engine is low. Such features cause precise controllability to be deteriorated, and thus, a scheme for enhancing precise control of the low pressure EGR system is highly desired in the field.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.